The present application hereby claims priority under 35 U.S.C. Section 119 on German Patent application number 101 31 556.2 filed Jun. 29, 2001, the entire contents of which are hereby incorporated by reference.
The invention generally relates to a device for pneumatic control. Preferably, it relates to one having a control piston carried in a cylinder that is mounted so as to be movable in the direction of a longitudinal piston axis. More preferably, it relates to one wherein a primary flow path that can be closed by the control piston passes through the cylinder and wherein the control piston is provided with a control surface that faces a control chamber of the cylinder and one that is designed to transmit a control force to the control piston.
Devices for pneumatic control are used in particular in the manufacture of blow-molded containers to be able to achieve delivery of one or more blow pressures that is coordinated with execution of the blow molding process.
In this type of container molding using blow pressure action, preforms of a thermoplastic material, for example preforms of PET (polyethylene terephthalate), are delivered to different processing stations within a blow molding machine. Typically, such a blow molding machine has a heater unit and a blow unit, in the vicinity of which the previously temperature treated preform is expanded by biaxial orientation to a container. The expansion is accomplished with the use of compressed air, which is introduced into the preform to be expanded. The process sequence of such an expansion is explained in DE-OS 43 40 291.
The basic structure of a blow station for molding containers is described in DE-OS 42 12 583. Options for temperature treatment of the preforms are explained in DE-OS 23 52 926.
Within the device for blow molding, the preforms and the blow-molded containers can be transported by means of various handling devices. One proven technique in particular is the use of transport mandrels onto which the preforms are placed. The preforms can also be handled with other carrying devices, however. For example, the use of grippers for handling preforms, and the use of expansion mandrels that can be introduced into a mouth area of the preform for the purpose of holding, are also among the available designs.
The aforementioned handling of the preforms takes place on the one hand as part of the so-called two-stage process, in which the preforms are first manufactured in an injection molding process, then are stored temporarily, before later being conditioned with respect to their temperature and blow molded into containers. On the other hand, application is also found in the so-called one-stage process, in which the preforms are appropriately temperature treated and then blow molded immediately after their production by injection molding and adequate hardening.
With regard to the blow stations employed, various embodiments are known. In blow stations that are arranged on rotating transport wheels, one frequently encounters mold supports that swing open in a book-like fashion. However, it is also possible to use mold supports that slide relative to one another or operate in other ways. In stationary blow stations, which are especially suitable for accommodating multiple cavities for container molding, plates that typically are arranged parallel to one another are used as molds.
The devices for pneumatic control used to supply compressed air to the blow stations are typically implemented as high pressure valves in which a control piston is positioned by a control pressure in such a way that the control piston either closes or opens a flow path for the blow pressure. To ensure a proper seal, the known control pistons are usually vulcanized with an elastomer in the area of a seal face.
A particular disadvantage of the known control pistons is that their design requires a different control pressure for holding the valve closed than for closing the valve. This results from the fact that different size areas of the control piston are subjected to the primary pressure in the closed and open states of the valve.
An object of an embodiment of the present invention is to design a device wherein the switching characteristics are improved.
An object may be achieved in accordance with an embodiment of the invention in that the control piston includes a piston shank whose cross-sectional area is dimensioned so as to be essentially equal to a cross-sectional area of a flow orifice of the primary flow path that faces an interior cylinder chamber and can be closed by the control piston.
Due to the essentially equal dimensions of the cross-sectional areas of the piston shank and the flow orifice of the primary flow path in the vicinity of the seal provided by the control piston, a result can be achieved wherein the forces required to close the pneumatic valve are of approximately the same order of magnitude as the forces for holding the valve in the closed position. This dimensioning makes it possible to significantly reduce the maximum control pressure and thereby achieve both a considerable reduction in switching time and a considerable increase in operating life. The reduction of the control pressure also brings about a reduction in noise so that smaller and thus more cost-effective noise dampers can be used with regard to the equipment.
Ease of assembly can be facilitated in that the control piston is designed in at least two parts.
In particular, in a multi-piece implementation of the control piston, the control piston may have a shank cap.
Simple assembly of the individual components of the control piston can be achieved in that the shank cap is attached via a snap-on connection in the vicinity of a piston shank of the control piston.
In particular, secure retention of the shank cap is achieved in that the piston shank has a groove for retaining the shank cap. However, it is also possible to provide the piston shank with an external profile that engages in a corresponding mating profile of the shank cap.
Positive fastening of the shank cap may also be supported by the shank cap engaging the groove with a ridge. Alternatively, the shank cap can also be equipped with a groove in which a corresponding ridge engages.
To avoid vulcanized-on seals, the shank cap may be made at least partially of plastic.
Especially favorable material properties can be achieved, for example, with the shank cap being made at least partially of PETP.
To improve sealing, at least one seal may be arranged in the vicinity of the piston shank.
To support the use of relatively small control pressures, provision may be made for the control surface to be dimensioned larger than the cross-sectional area of the piston shank.
For a compact design implementation, the control surface may be positioned in the vicinity of a piston head facing away from the shank cap.
Providing the piston head with at least one seal also contributes to avoiding pressure losses.
In accordance with one embodiment of the invention, it is intended for both the piston shank and the piston head to be made at least partially of metal.
The use of special guide bands on the control piston can be avoided in that both the piston shank and the piston head are made at least partially of plastic.
Particularly favorable material properties can be achieved in that both the piston shank and the piston head are made at least partially of PETP.
To assist in the generation of the actuating force for the control piston, it is proposed that a control chamber for delivery of a control pressure be at least partially delimited by the control surface.
Especially in a metallic implementation of the control piston, it is intended that the control piston be provided with a damping element for end point damping.